1. Field of the Invention
This invention relates to automotive power transmissions, and in particular to infinitely-variable, hydrostatically-based transmissions having a lockup capability in one or more ranges.
2. Description of the Related Art
Current automatic transmissions used in the larger full-sized automobiles lighter duty trucks and passenger vans generally feature some sort of torque converter lockup or overdrive. The transmission goes into the lockup mode or overdrive range when selected by the operator, and after attaining the predetermined speed and engine torque parameters. When the transmission is in lock up mode, the torque converter is by-passed and the engine is mechanically coupled to the vehicle drive wheels. This lockup or overdrive incorporates a higher drive ratio than is achieved without it, which makes this transmission ideal for extended highway driving. As well, because of the direct mechanical coupling between the engine and the drive wheels when in overdrive, this transmission approaches the fuel economy levels of conventional manual shift transmissions for long distance highway driving, especially where heavy loads or frequent steep terrain do not cause it to go out of the overdrive mode a significant amount of the time. Of course, in city driving the vehicle speeds attained are not usually sufficient for the lockup mode to engage; hence, similar efficiencies cannot be achieved in this instance.
In heavier-duty double-axle trucks, automatic transmissions with multiple lockup modes are sometimes used. In the large highway transport tractor units, as well as in the larger four-wheel-drive farm tractors, hydraulically-clutched, multi-ratio mechanical transmissions, sometimes incorporating several dozen discreet drive ratios, are generally used. This multiplicity of mechanical drive ratios, coupled with hydraulically-controlled automatic shifting and clutching, makes it possible to completely dispense with a torque converter. What these machines rely on instead of a smooth stepless change of ratios during acceleration is the fact that the adjacent drive ratios are so close together that the virtually instantaneous shifting results in such small incremental changes in engine RPM that the engine and drive train components are not unduly stressed. However, in order to achieve the large number of discreet drive ratios required, the transmission must be very large and, consequently, very expensive.